Single crystal beryllium oxide growth from calcium oxide-beryllium oxide melts

ABSTRACT

A process for growing large single crystals substantially free of surface inclusions within several days is disclosed. This process involves the concept of promoting further growth on seed crystals from a melt composition of suitable oxides which will form a eutectic. Growth occurs as the crystal seed is rotated and pulled slowly from the melt composition. The temperature decrease of said melt is such that the temperature can be defined by the liquidus curve of the melt composition phase diagram. Growing single crystal beryllium oxide at a temperature of about 1,980* C from a melt composition having 56 weight percent beryllium oxide and 44 weight percent calcium oxide is an example of this process.

United States Patent [1 1 Elkins et al. I

[ Oct. 30, 1973 [75] Inventors: Perry E. Elkins, Santa Ana; Stanley B.Austerman, Villa Park, both of Calif.

[73] Assignee: North American Rockwell Corporation, El Segundo, Calif.

[22] Filed: Nov. 3, 1971 [211 App]. No.: 195,365

Steele et al. 23/304 3,490,959 1/1970 Krock et al. 148/22 3,595,8037/l97l Dugger 23/305 3,650,702 3/1972 Swets 23/300 PrimaryExaminer-Norman Yudkoff Assistant Examiner-R. T. Foster Attorney-15. LeeHumphries et al.

[57] ABSTRACT A process for growing large single crystals substantiallyfree of surface inclusions within several days is disclosed. Thisprocess involves the concept of promoting further growth on seedcrystals from a melt [52] US. Cl. 23/301 SP, 23/305, 423/122,

423/624 composmon of suitable modes which w1ll form a eu- 511 110. c1B01 j 17 20 tectic- Gmwth as the crystal Seed is mated [58] Field ofSearch 23/305 301 R 300 Pulled slwly the The 23/301 SP, 304;123/624ature decrease of said melt is such that the tempera- 1218/21): ture canbe defined by the liquidus curve of the melt composition phase diagram.Growing single crystal be- [56] References Cited ryllium oxide at atemperature of about 1,980 C from UNITED STATES PATENTS a meltcomposition having 56 weight percent beryllium oxide and 44 weightpercent calcium oxide is an 2,848,310 8/1958 Remeika 23 305 example fthis Proms, 3,234,135 2/1966 Ballman et al 23/305 3,341,302 9/1967Flanigen et al. 23/301 R 8 Claims, 1 Drawing Figure I l 22oo 7 T 7 19 0"l800 LIQUID +C0O o I v I. I400 I 0 1o 20 3o 40 so 56 so so I00 BeOPAIENTEnumao I913 3.7683 3 IZOO I000 I l I J I0 4-0 505660 so I00 & v g

INVENTORS BY STANLEY 'B. AUSTERMAN ATTORNEY PERRY E ELKINS SINGLECRYSTAL BERYLLIUM OXIDE GROWTH- FROM CALCIUM OXIDE-BERYLLIUM OXIDE MELTSBACKGROUND OF THE INVENTION 1. Field of the Invention This inventionrelates to crystal growing and more particularly to a method of growingsingle crystals from a melt composition.

2. Description of Prior Art The traditional Czochralski method ofgrowing crystals from a liquid melt composition is well known. In thisprocess the material from which a single crystal is to be grown isplaced into a crucible and melted to form a liquid melt. The material ofthis melt is usually of substantially the same material as the seedcrystal. The temperature of the liquid melt is maintained 1 to 2 abovethe melting point of the material. A single crystal seed is lowered intothe liquid. Upon contact the liquid starts to solidify onto the seed.The seed is then slowly pulled from the liquid while being rotated.Continued solidification of the liquid onto the seed results in thegrowth of a single crystal.

The Czochralski method of growing single crystals of, for example,beryllium oxide (BeO) increases the danger of crystal cracks in the BeO.In this example, a BeO melt composition must be maintained at atemperature of about 2,450 C, the melting point of BeO. As the singlecrystal BeO is cooled below 2,050 C, the reported Alpha-Beta phasetransition of BeO, the crystal damage that normally accompanies thisphase transformation can occur thereby rendering a low yield of usuableBeO crystals.

The Alpha-Beta phase transition of BeO is also known as a crystalstructure phase transformation from high temperature tetragonal to lowtemperature hexagonal crystal structure.

Another disadvantage of growing in particular BeO single crystals by theCzochralski method is that there is presently a very limited number ofsuitable crucible materials, eg tungsten, molybdenum and iridium, toaccommodate molten BeO at its melting temperature of 2,450 C.

The flux growth method is another well known process for growing singlecrystals. The flux growth method differs from the Czochralski process inthat the material that is to be grown is dissolved in a suitable flux,such as lithium polymolybdate. In the flux method a crucible containinga liquid solution of flux and said material is placed in a furnace.Single crystals are precipitated by lowering the temperature of thefurnace causing the flux solution to become supersaturated with thematerial. As the material begins to precipitate out of the solutionsmall single crystals nucleate and grow on the crucible floor and walls.

The primary obstacle in growing single crystals by the flux process isthat presently good quality crystals cannot consistently be grown. Theflux process produces a low yield of usuable crystals as a result offlux inclusions and surface irregularities in the crystals. Anotherdisadvantage of the flux process is that it takes several months to growsingle crystals which are of any useful size.

SUMMARY OF THE INVENTION This invention relates to a process of growingsingle crystals by promoting further growing on a single crystalseedfrom a melt composition. The melt composition contains a mixture ofselected oxides which can form a eutectic composition. The advantage ofsuch a melt composition is that it melts at a lower temperature than amelt composition of only the material to be crystallized. Thetemperature of the melt composition is lowered to the liquidustemperature, that is, where solid material with the same composition asthe seed comes out of the solution. When the temperature of the meltcomposition is stabilized at the liquidus temperature, a seed crystal islowered into the surface of the melt. Material of the same type as theseed precipitates from the melt composition and grows on the seedcrystal. Growth of the seed material from the melt composition occurswhile the seed is rotated and slowly pulled from the melt. During thegrowth process on the crystal, the seed type material concentration inthe melt is reduced; and the melt temperature is lowered such that thetemperature can be defined by the liquidus curve of the respective meltcomposition phase diagram. After 10 to 15 hours of growth, a largesingle crystal boule substantially free of inclusions is recovered fromthe melt.

An example of a preferred embodiment of this invention is to grow aberyllium oxide, BeO, single crystal from a melt composition containing56 weight percent BeO and 44 weight percent CaO at a temperature ofl,980 C.

Other objects and advantages of this invention will be apparent from thefollowing detailed description wherein a preferred embodiment of thepresent invention is clearly shown.

BRIEF DESCRIPTION OF THE DRAWING The drawing shows a phase diagram of aBeO-CaO mixture.

DETAILED DESCRIPTION This invention involves a process of growing largesingle crystals by promoting the growth on seed crystals from a meltcontaining a mixture of suitable oxides. As shown in the drawing, theBeO-CaO melt will be used to describe the growth of beryllium oxide,BeO, crystals by this process. BeO single crystals can be grown fromBeO-CaO melt compositions containing 40 or more weight percent BeO.Preferably, the mixture should melt below 2,050 C, the reportedAlpha-Beta phase transition temperature of BeO which usually causescrystal damage. A melt composition having 56 weight percent BeO and 44weight percent CaO as shown in the drawing is such a melt composition.This melt composition does not exceed the Alpha-Beta phasetransformation temperature of BeO and therefore is not subjected to therisk of crystal destruction. It should also be notedthat by utilizingthis type of mixture the melt composition temperature is substantiallybelow 2,450 C, the melting temperature of BeO.

The mixture containing, for example, 56 weight percent BeOand 44 weightpercent CaO is heated to form a melt in an iridium crucible at aboutl,980 C, the liquidus temperature 14 where solid BeO seed material comesout of the melt.

After the melt composition is stabilized at the liquidus temperature 14,a BeO seed crystal is lowered into position below the liquidustemperature 14 in the area of the BeO seed results in BeO materialcoming out of the super-saturated melt and precipitating onto the singlecrystal BeO seed.

The growth process of BeO onto the seed continues while the seed isrotated and slowly pulled from the melt until equilibrium isre-established between the seed and the melt. During the Eco growth onthe seed, the concentration of the seed material in the melt is reduced.The melt temperature is then lowered to conform to the liquidus curve 18of the BaO-CaO phase diagram as illustrated in the drawing.

The lowering of the melt interface temperature is critical for growth toproceed. That is, when BeO is removed from the melt, the meltcomposition becomes richer in the other oxide component CaO and theliquidus temperature 14 decreases. If the temperature is lowered at tooslow a rate during the pulling of the crystal seed, the melt-solidtemperature represented by dashed arrow 16 becomes higher than theinstantaneous liquidus temperature 14 of the melt composition resultingin the crystal breaking away from the melt causing all growth to cease.

Conversely, if the temperature is lowered at too rapid a rate for thepull rate of the crystal, the melt-solid temperature 25 tends to becomesubstantially lower than the liquidus temperature 14 as illustrated. Theresult is excessive super-saturation resulting in spontaneous nucleationof BeO throughout the melt and growth on the seed becomespolycrystalline.

The preferred condition is to couple the pull rate of the crystal seedand temperature decrease such that the actual melt-solid temperature isonly 2 or 3 lower than the temperature on the liquidus line 18. Thus,conditions at the crystal-melt interface would follow the liquidus curve18 of the phase diagram during the growth run. As the system is furthercooled from the liquidus temperature 14 towards the eutectic temperaturel0, BeO continues to crystallize until reaching the eutectic temperature10, the temperature at which the eutectic mixture 12 is formed.

After a hour growth run utilizing the foregoing process, a large BeOsingle crystal substantially free of surface inclusions is pulled fromthe melt composition.

This invention is applicable for growing single crystals in the form ofboules or thin wafers. The foregoing process is utilized when growingbowles as contrasted to thin wafers. When growing thin wafers, the seedcrystal is not pulled from the melt, rather the melt temperature islowered slowly at a rate of about 10 to 20 C per hour. A preferred rateis 15 C per hour. During the temperature decrease BeO will come out ofthe melt composition and deposit on the seed. Large wafers have beengrown in 10 hours on the surface of the melt. The single crystal BeOwafer can be recovered from the melt by pulling the pull rod out withthe attached crystal at a rapid rate.

EXAMPLE I A mixture containing 50 weight percent BeO and 50 weightpercent CaO is melted in an iridium crucible at a temperature of l,900C. The temperature of the melt is lowered to about l,800 C, the liquidustemperature of the melt. A BeO seed crystal is lowered into the melt byconventional mechanical means. The melt in the area of the seed wascooled below the liquidus temperature causing BeO to come out of themelt and precipitate onto the BeO seed. The BeO seed continued to growwhile it was rotated and slowly withdrawn from the melt. The temperatureof the melt was decreased during the growth of the Eco seed to followthe liquidus line of the phase diagram of the BeO-CaO system. A largeBeO single crystalboule weighing 7% grams substantially free of surfaceinclusions was grown in 24 hours.

EXAMPLE [I A mixture containing 56 weight percent BeO and 44 weightpercent CaO was melted in an iridium crucible. The temperature of themelt was lowered to about 1,980 C, the liquidus temperature of the meltcomposition. A BeO seed crystal was lowered into the upper portion ofthe melt composition by conventional mechanical means.

The melt region near the crystal seed was cooled to below the liquidustemperature. BeO came out of the melt and precipitated onto the Ecoseed. The melt temperature was lowered slowly at a rate of about 10 'to20 C per hour. After 10 hours a large wafer was grown on the surface ofthe melt. The single crystal BeO wafer was then removed from the melt bypulling out the seed rod and the attached seed crystal at a rapid rate.

This invention permits BeO crystals to be grown from other melts such asstrontium oxide, SrO,-beryllium oxide, BeO, and barium oxide, BaO,-beryllium oxide, BeO. CaO crystals can also be grown from a calciumoxide, CaO, -beryllium oxide, BeO melt where the melt composition isless than 40 weight percent BeO and the mixture of CaO-BeO will meltbelow the CaO phase transformation temperature. By utilizing thismethod, many oxides can be grown in crystal form such as strontium oxidecould be grown from SrO-BeO melts or titanium dioxide, TiO could begrown from TiO -BeO melts.

A third oxide can be added to a mixture of BeO-CaO to improve crystalquality. Oxide such as Ba, Sr, Mg, Al, Na, K, Y, La, and the rare earthelements may shift the eutectic composition and the melting temperaturethereof and thereby improve crystal quality.

We claim:

1. A process for growing beryllium oxide single crystals comprising thesteps of providing a mixture containing at least beryllium oxide andcalcium oxide, wherein the concentration of beryllium oxide in saidmixture exceeds the beryllium oxide concentration required for aeutectic composition of beryllium oxide-calcium oxide, heating saidfirst mixture to form a melt,

lowering the temperature of said melt to its liquidus temperature, and

placing a beryllium oxide seed crystal into the surface of said meltwhereby beryllium oxide from said melt grows on said seed crystal.

2. The process recited in claim 1 whereby said seed crystal is slowlyrotated and slowly withdrawn from the melt during growth to form a bouleabout the seed crystal.

3. The process recited in claim 1 wherein the melt temperature islowered at a rate of about 15 C per hour.

4. The process recited in claim 1 wherein said first mixture includes athird oxide compound.

while beryllium oxide from said melt grows on said seed crystal to causethe temperature of the melt to substan tially follow the liquidustemperature curve of the melt towards the eutectic temperature.

' 8. The process recited in claim 1 wherein a thin wafer of singlecrystal material is formed on, the surface of said melt adjacent to saidseed crystal.

2. The process recited in claim 1 whereby said seed crystal is slowly rotated and slowly withdrawn from the melt during growth to form a boule about the seed crystal.
 3. The process recited in claim 1 wherein the melt temperature is lowered at a rate of about 15* C per hour.
 4. The process recited in claim 1 wherein said first mixture includes a third oxide compound.
 5. The process recited in claim 4 wherein said third oxide compound is selected from the group consisting of barium, strontium, magnesium, aluminum, sodium, potassium and the rare earth elements.
 6. The process recited in claim 1 wherein the beryllium oxide concentration in said first mixture varies from 40 to 70 weight percent.
 7. The process recited in claim 1 including the step of continuously lowering the temperature of said melt while beryllium oxide from said melt grows on said seed crystal to cause the temperature of the melt to substantially follow the liquidus temperature curve of the melt towards the eutectic temperature.
 8. The process recited in claim 1 wherein a thin wafer of single crystal material is formed on the surface of said melt adjacent to said seed crystal. 